ATP-induced membrane currents in ameboid microglia acutely isolated from mouse brain slices

Microglial cells were harvested from the surface of corpus callosum slices acutely isolated from the brain of neonatal (five- to seven-day-old) mice. Transmembrane ionic currents were measured employing a standard whole-cell voltage-clamp technique. The extracellular application of 1 mM ATP triggered the generation of a complex membrane current comprising three components: (i) an initial fast inward current which had a reversal potential at about -20 to -15 mV; (ii) this initial component was followed by a steady-state inward current with reversal potential about -50 to -40 mV; and (iii) a delayed inward current with a reversal potential close to 0 mV. The first two components (fast and steady-state) had an activation threshold at 10 microM ATP, and 100 microM ATP evoked an almost maximal response. In contrast, the third component of ATP-induced inward membrane current could be observed only while 1 mM ATP was applied. The increase in concentration of tetra-anionic form of ATP (ATP4-) by removal of divalent cations from the bath solution substantially lowered the activation threshold for the delayed component of ATP-induced membrane current; conversely, lowering the ATP4- concentration (by replacing Ca2+ with Mg2+) resulted in its disappearance. These results suggest that ATP4- acts as a true agonist for the activation of the delayed ATP-induced membrane current. We conclude that microglial cells express several purinoreceptor subtypes. The activation of these receptors might play a role in intracellular signal transduction in brain microglia.